libs/gnttab: introduce grant copy interface

libs/gnttab: introduce grant copy interface

In a linux part an ioctl(gntdev, IOCTL_GNTDEV_GRANT_COPY, ..)
system call is invoked. In mini-os the operation is yet not
implemented. For the OSs that does not implement gnttab the
call of the grant copy operation causes abort.

Signed-off-by: Paulina Szubarczyk <paulinaszubarczyk@gmail.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
[ wei: modify this patch to use BUILD_BUG_ON in xen-tools/libs.h ]
Signed-off-by: Wei Liu <wei.liu2@citrix.com>